Atomic layer deposition (ALD) is an enabling technology for next generation conductor barrier layers, high-k gate dielectric layers, high-k capacitance layers, capping layers, and metallic gate electrodes in silicon wafer processes. ALD has also been applied in other electronics industries, such as flat panel display, compound semiconductor, magnetic and optical storage, solar cell, nanotechnology and nanomaterials. ALD is used to build ultra thin and highly conformal layers of metal, oxide, nitride, and others one monolayer at a time in a cyclic deposition process. Oxides and nitrides of many main group metal elements and transition metal elements, such as aluminum, titanium, zirconium, hafnium, and tantalum, have been produced by ALD processes using oxidation or nitridation reactions. Pure metallic layers, such as Ru, Cu, Ta, and others may also be deposited using ALD processes through reduction or combustion reactions.
A typical ALD process uses sequential precursor gas pulses to deposit a film one layer at a time. In particular, a first precursor gas is introduced into a process chamber and produces a monolayer by reaction at surface of a substrate in the chamber. A second precursor is then introduced to react with the first precursor and form a monolayer of film made up of components of both the first precursor and second precursor, on the substrate. Each pair of pulses (one cycle) produces exactly one monolayer of film allowing for very accurate control of the final film thickness based on the number of deposition cycles performed.
As semiconductor devices continue to get more densely packed with devices, channel lengths also have to be made smaller and smaller. For future electronic device technologies, it will be necessary to replace SiO2 and SiON gate dielectrics with ultra thin high-k oxides having effective oxide thickness (EOT) less than 1.5 nm. Preferably, high-k materials should have high band gaps and band offsets, high k values, good stability on silicon, minimal SiO2 interface layer, and high quality interfaces on substrates. Amorphous or high crystalline temperature films are also desirable. Some acceptable high-k dielectric materials are listed in Table 1. Among those listed, HfO2, Al2O3, ZrO2, and the related ternary high-k materials have received the most attention for use as gate dielectrics. HfO2 and ZrO2 have higher k values but they also have lower break down fields and crystalline temperatures. The aluminates of Hf and Zr possess the combined benefits of higher k values and higher break down fields. Y2O3 has high solubility of rare earth materials (e.g. Eu+3) and is useful in optical electronics applications.
TABLE 1Dielectric properties of ALD high-k gate materialsBreak downField EBDEOT(MV/cm @CrystallineMaterialK(@ 5 nm film)1 μA/cm2)Temp (° C.)HfO213-171.31-5400-600Al2O3 7-92.443-8 900-1000ZrO2200.981 <300 *HfxAlyOz 8-201.22N/A900ZrxAlyOz 8-201.22N/A975Y2O312-151.444<600 Ta2O523-250.810.5-1.5500-700NbxAlyOz 82.445N/AHfxSiyOzN/AN/AN/A800TaxTiyOz27-280.711N/AAl2O3/HfO2N/AN/AN/AN/AAl2O3/TiO2 9-181.445-7N/A* as a function of film thickness
Transition metals and metal nitrides may be used as diffusion barriers to prevent inter-diffusion of metal and silicon in IC devices. These barrier layers are only a few nm in thickness, and are conformal in trenches and vias. Table 2 shows some properties of ALD grown barriers. Desirable properties include low growth temperature (<400° C.) and low film resistivity. For example, Ta/TaN and W/W×N are preferred as copper diffusion barrier systems. ALD metal thin layers, such as Ru, Cu, Pt, and Ta, have also been deposited for use as barrier and seed layer applications.
TABLE 2Film properties of ALD nitride barrier layer materialsMetalOtherGrowth TempResistivityFilmprecursorprecursors(° C.)(μΩ*cm)TaNTaCl5Zn + NH3400-500900TaNTaCl5H/N plasma300-400300-400TaN(C)TBTDETNH3250N/ATaN(C)TBTDETH plasmaN/A250TaNxTaF5H/N plasma250104-103Ta3N5TaCl5NH3400-500105-104W2NWF6NH3330-5304500 TiNTiCl4NH3500250TiNTiCl4Zn + NH3500 50TiNTiI4NH3400-500380-70 TiNTiCl4Me2NNH2350500TiNTEMATNH3160-320600(post annealed)TiNTi(NMe2)4NH31805000 
ALD is an advanced deposition method for high density memory devices when highly conformal and high aspect ratio deposition of high-k dielectric materials and its liners is needed. High-k oxides listed in Table 1, such as Al2O3, as well as ferroelectric materials, such as BST, PZT, and SBT layers, have been used as capacitor dielectrics in memory devices.
Several types of traditional vapor phase deposition precursors have been tested in ALD processes, including halides, alkoxides, β-diketonates, and newer alkylamides and cyclopentadienyls materials. Halides perform well in ALD processes with good self-limiting growth behaviors, but are mostly high melting solids that require high source temperatures. Another disadvantage of using solid precursors is the risk of particle contamination to the substrate. In addition, there is an issue of instability in flux or dosage associated with the solid precursors. Alkoxides show reduced deposition temperatures in ALD processes, but can decompose in the vapor phase leading to a continuous growth process instead of ALD. β-diketonates are used in MOCVD processes and are generally more stable towards hydrolysis than alkoxides. However, they are less volatile and require high source and substrate temperatures. A mixed ligand approach with β-diketonates and alkoxides has been suggested to improve stability of alkoxide MOCVD precursors. Examples are Zr(acac)2(hfip)2, Zr(O-t-Pr)2(thd)2. In addition, metal nitrate precursors, M(NO3)x, alkylamides, and amidinates, show self-limiting growth behavior with very low carbon or halide contamination. However, the stability of nitrates and amides is an issue in production and many cyclopentadienyls are in solid forms.
In general, ALD precursors should have good volatility and be able to saturate the substrate surface quickly through chemisorptions and surface reactions. The ALD half reaction cycles should be completed within 5 seconds, preferably within 1 second. The exposure dosage should be below 108 Laugmuir (1 Torr*sec=106 Laugmuir). The precursors should be stable within the deposition temperature windows, because un-controllable CVD reactions could occur when the precursor decomposes in gas phase. The precursors themselves should also be highly reactive so that the surface reactions are fast and complete. In addition, complete reactions yield good purity in films. The preferred properties of ALD precursors are given in Table 3.
TABLE 3Preferred ALD precursor propertiesRequirementClassPropertyRangePrimaryGood volatility>0.1 TorrPrimaryLiquid or gasAt room temperaturesPrimaryGood thermal stability>250° C. or >350° C.in gas phasePrimaryFast saturation<5 sec or <1 secPrimaryHighly reactiveComplete surfacereactive cyclesPrimaryNon reactive vola-No product andtile byproductreagent reactionSecondaryHigh growth rateUp to a monolayera cycleSecondaryLess shield effectFree up un-occupiedfrom ligandssitesSecondaryCost and purityKey impurity:H2O, O2SecondaryShelf-life>1-2 yearsSecondaryHalidesFree in filmsSecondaryCarbon<1% in non carboncontaining films
Because of stringent requirements for ALD precursors as noted in Table 3, new types of ALD precursors are needed that are more stable, exhibit higher volatility, and are better suited for ALD. However, the cost of developing new precursors is a significant obstacle. In this light, the prior art related to chemical vapor deposition (CVD) processes provides some useful background information.
Direct liquid injection methods have been used in many vapor phase deposition processes. For example, U.S. Pat. No. 5,376,409 describes a method of delivering solid precursors that have been dissolved in an appropriate solvent for use in chemical vapor deposition (CVD) techniques. U.S. Pat. No. 5,451,260 describes a method for providing a liquid precursor solution for direct injection using an ultrasonic nozzle for CVD techniques. Beach, et al., in “MOCVD of very thin films of lead lanthanum titanate”, MRS symposium proceedings, 415, 225-30 (1996) set forth a CVD method using multiple precursors dissolved in a single solution. Choi, et al., “Structure stability of metallorganic chemical vapor deposited (Ba, Sr)RuO3 electrodes for integration of high dielectric constant thin films”, Journal of the Electrochemical Society, 149(4), G232-5 (2002), describes a CVD method using liquid injection of a multiple component solution. Zhao, et al., “Metallorganic CVD of high-quality PZT thin films at low temperature with new Zr and Ti precursors having mmp ligands”, Journal of the Electrochemical Society, 151(5), C283-91 (2004) discusses another CVD method using a multiple precursor solution liquid delivery system. As noted, each of these references discuss CVD techniques and are interesting only for the discussion of various precursor materials, including solid precursors dissolved in appropriate solvents.
There is also some prior art background material relating to ALD processes. Cho, et al., “Atomic layer deposition (ALD) of Bismuth Titanium oxide thin films using direct liquid injection (DLI) method”, Integrated Ferroelectrics, 59, 1483-9, (2003), reports on the use of solid precursors dissolved in a solvent. However, no information is provided concerning the delivery and deposition methods.
US published patent application 2003/0056728 discloses a pulsed liquid injection method in an atomic vapor deposition (AVD) process, using a precursor in liquid or dissolved form. The liquid dose is too large for ideal ALD operation. Min, et al., “Atomic layer deposition of Al2O3 thin films from a 1-methoxy-2-methyl-2-propoxide complex of aluminum and water”, Chemistry Materials (to be published in 2005), describes a liquid pulsing method for solution precursors, where again the liquid dose is too large for ideal ALD operation. In fact, using liquid pulse to achieve monolayer coverage is very difficult, because in an ALD operation, the pulse width of a vapor phase reactant is 1 second or less. One issue is that the shape of a vaporized liquid pulse is distorted in time space and sharp leading and tailing edges of the liquid pulse can be lost after vaporization. It is therefore difficult to synchronize two well separated reactants to perform self-limiting and sequential ALD growth. The liquid pulse methods described in the two references above do not represent true ALD processes but rather variants of CVD processes.
US published patent application 2004/0079286, describes a two-phase delivery system for ALD wherein both vapor and liquid phase coexist in a vaporizer after liquid injection. This process will not work for solution based precursors or multi-component mixtures where material separation would occur.
There remains a need in the art for improvements to ALD precursors and methods of using such precursors in ALD processes.